Power conservation is a primary concern for modern communication receivers, such as selective call receivers and the like. Conventional communication protocols, including the Post Office Code Standardisation Advisory Group or POCSAG protocol, have well known provisions for conserving power at the receivers. Typically, these include providing a predetermined signal during predetermined time intervals in a periodic fashion that can be monitored by the receivers. At those time intervals that monitoring is not required, the receivers can conserve power by shutting off non-essential circuits and functions. In this way, the receivers can extend the life of their power source, e.g. their battery.
FIGS. 1A, 1B, and 1C, illustrate the POCSAG signalling protocol. Referring to FIG. 1A, every new transmission comprises a preamble 102 followed by at least one batch 104, and often a number of batches. Each batch, as shown in FIG. 1B, comprises a predetermined synchronization code word (hereinafter "SYNC CW") followed by eight frames, numbered zero through seven. Each frame includes two phases, 112, 114, that each can contain address information, message information, or control information, as defined by the POCSAG protocol.
A receiver following the POCSAG protocol can synchronize to a new transmission by monitoring the preamble 102 of the new transmission. Subsequently, the receiver synchronizes to the SYNC CW of each batch and typically then monitors an assigned frame within the batch for address information. To conserve power, the receiver may avoid monitoring other frames in the batch. The next monitoring time interval occurs at the SYNC CW of the following batch. Therefore, by only monitoring the SYNC CW of each batch within a transmission, the receiver maintains synchronization to the transmission while conserving power. Once the SYNC CW is detected the receiver can monitor the assigned frame within the batch for address information, and shut off non-essential circuits and functions at other times.
FIG. 1C illustrates a feature of the POCSAG protocol that provides receivers the capability to detect a new transmission while monitoring the present transmission. The length of the preamble 102 is equivalent to the combined length of two SYNC CW's and eight frames 120. Receivers that periodically monitor for the SYNC CW of a batch within the present transmission are guaranteed to monitor at least a portion of the preamble of a subsequent new transmission. In this way, the receivers can detect the new transmission by the presence of the preamble 102. The receivers can then synchronize their detection circuits to the new transmission and search for the SYNC CW of the first batch of the new transmission to begin monitoring for address information therein.
A known method of further conserving power within a receiver comprises monitoring a portion of a periodic predetermined signal within a transmitted signal, such as a portion of the SYNC CW in the POCSAG protocol. When a first portion of the SYNC CW is detected within an acceptable bit error rate, non-essential circuits and functions of the receiver can be shut off for a second portion of the SYNC CW. Although this has the advantage of significantly conserving power, detecting the first portion of the SYNC CW under certain signalling conditions may be less reliable than detecting the full SYNC CW. Specifically, a receiver may falsely determine that it maintains synchronization with the present transmission, while delaying detection of a subsequent new transmission. In a worst case, the receiver may not synchronize to the new transmission in time to detect the first batch thereof. Consequently, the receiver may miss its address information and any associated message information. This can be very frustrating to a user of the device. In a medical or governmental communication system, the consequences of not receiving a message may be grave and devastating.
Thus, what is necessary is a method and apparatus for conserving power and maintaining synchronization by monitoring a portion of a periodic predetermined signal within the present transmission, while re-synchronizing to a new transmission in time to reliably detect the first batch thereof.
In carrying out one form of this invention, there is provided a method for synchronizing a receiver to a transmitted signal, the transmitted signal having a first predetermined signal for indicating a start of a new transmission to the receiver and having a second predetermined signal inserted at periodic sampling time intervals for synchronizing the receiver to the transmitted signal, the method comprising the following steps: synchronizing to the transmitted signal by detecting the first predetermined signal followed by the second predetermined signal, and establishing subsequent periodic sampling time intervals therefrom, conserving power in the receiver during a second portion of one of the periodic sampling time intervals following a detection of a first portion of the second predetermined signal during a first portion of the one of the periodic sampling time intervals, and searching for a portion of the first predetermined signal during the second portion of the one of the periodic sampling time intervals following a lack of detection of the first portion of the second predetermined signal during the first portion of the one of the periodic sampling time intervals.